change attribute type hints form num to double

The current version is not running in darts checked mode because many attributes are typed as num an contain as integers as well as doubles as values but some attributes are typed as doubles.
So when one class which num-attributes gets in touch with another class with more strictly typed double-attributes there will be conflicts.

To be consistent I changed all attributes to be typed as double. In terms of BC this is a little break but the current code does not work in checked mode anyway. With checked mode disabled the BC break is not noticeable.